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High Yield Synthesis of Carbon Nanotubes

Final Activity Report Summary - HYS OF CNTS (High Yield Synthesis of Carbon Nanotubes)

Within the carbon nanotubes (CNT) project framework high yield synthesis methods were proposed and mechanistic investigations of CNT nucleation and growth, as well as electrical applications of individual and composite CNTs were carried out.

We firstly proposed a novel substrate chemical vapour deposition (CVD) method for the CNT high yield production and built the reactor based on continuous feeding of catalyst powder using a screw feeder. We proposed and utilised for the first time one of the basic building materials for the CNT growth, cement, originally containing catalyst material. This allowed us to avoid many time-consuming steps of the catalyst-support preparation. We chose acetylene as the main carbon source because of its low decomposition temperature and cost. The experimental investigations were carried out with the powder feeding rate of 30 g/hour in the temperature range of 500 to 600 degrees Celsius. Based on scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman, X-ray diffraction (XRD) and TG measurements a high efficiency of the reactor was shown for the high yield, up to 15 g/hour, and low temperature synthesis. Our investigations of electrical resistivity demonstrated the essential improvement of electrical conductivity of concrete which was made of the CNT-cement system.

We also built and tested a lab-scale reactor based on the ferrocene vapour decomposition. A few problems, such as wall contaminations, were found and solved. The pilot-scale ferrocene-CO reactor with an estimated CNT yield of about 20 g/day was designed according to computational fluid dynamic calculations.

Moreover, by utilising the method which was based on the ferrocene vapour decomposition as well as a hot wire generator method in the atmosphere of carbon monoxide we synthesised and discovered a novel carbon material, NanoBuds, where single-walled CNTs and fullerenes were combined into a single hybrid structure, showing promising field-emission properties.

When studying gas-phase synthesis of CNTs and their post-growth behaviour, we discovered the phenomenon of spontaneous CNT charging. This allowed us to elaborate a novel method for the room temperature deposition of individual CNTs onto any substrate. The mechanistic investigations of the growth of CNTs in the presence of carbon monoxide and iron-containing particles in a nitrogen atmosphere on the basis of aerosol measurements showed the formation of positively and negatively charged (up to 99 %) single-walled CNT bundles. On the basis of the analysis of the laser desorption-ionisation time of flight (LDI-TOF) experimental data it was proposed that the positive charging of CNTs occurred because of the electron emissions, while negative charging was caused by emission of impurities from the CNT surface.

We furthermore synthesised single-walled CNTs with very large diameters (up to 11 nm, which was the largest ever synthesised single-walled CNT) by a substrate CVD method using carbon monoxide and iron nanoparticles deposited on the TEM grid and studied the CNT deformation due to the interaction with substrates. TEM examination of CNTs at different angles revealed that the deformation of individual CNTs due to the interaction with a surface occurred at diameters larger than 2.7 nm.

The mechanistic studies of the formation of single-walled CNTs were carried out in two different aerosol methods. They were based on introduction of pre-formed catalyst particles into conditions leading to CNT synthesis, called hot wire generator reactor, and on chemical way of catalyst synthesis, namely ferrocene reactor. The vital role of etching agents such as CO and H2O in CNT formation was shown. The carbon nanotube growth was found to occur at a temperature of around 900 degrees Celsius in the heating section of the reactor by in situ sampling and the growth rate was calculated greater than 1 100 nm/s. A detailed analysis of possible processes during carbon nanotube formation revealed heptagon transformation as a limiting stage. A mechanism for CNT formation in both experimental systems was proposed.

Finally, the behaviour of motion of nanoparticles in gases, which was necessary to develop methods for synthesis and assembly of nanostructures, was analysed.